Moms, Place, and Low Birth Weight, Part 3: Place and Race Together

This is the third in a three-part blog series examining correlates of low birth weight in babies born in 2010, 2011, and 2012 in Detroit, Wayne County outside of Detroit, Oakland, and Macomb counties. Low birth weight (LBW), defined as 2500 grams or less1, is a significant contributor to Detroit’s alarmingly high infant mortality rate.2 We offer this analysis in the belief that a better understanding of factors influencing birth weight can help reduce the mortality rate.

The first blog post looked at the associations between a baby’s birth weight and the mother’s age, education, marital status, ethnicity, and race; the level of prenatal care she received; and the area of residence for women within the city of Detroit. That analysis showed that birth weight was related to the mother’s characteristics for women living in Detroit, although the strength of the relationship depended on the characteristic.

The second blog post compared the findings for Detroit to those for the “Metro Region” ­­– defined as Wayne County outside of Detroit, Oakland County, and Macomb County — on the same characteristics, in effect asking, “Does place matter?” We found that by including place of residence along with the mother’s characteristics we gained an even better understanding of demographic influences on birth weight. So yes, place does matter for birth weight for many of the characteristics.

In this third blog post we take the examination of associations between mothers’ demographic characteristics and their children’s birth weight even further by adding another layer to the analysis: the mother’s race. Because a major difference between Detroit and Metro Region women giving birth during this period is the racial composition of the two groups, and we know that race can have major implications for an individual’s opportunities and well-being, we investigate the effects of combinations of place and race on LBW rates for the various characteristics of the mother.

Source of Data

The data for this study came from birth certificate records of babies born in 2010, 2011, and 2012 with the mother’s residence in Wayne, Oakland, or Macomb County. The first blog post in this series described the source in detail. For the present analysis, we included only those women who were listed on the birth certificate as either Black/African American or White, and created four place-race groups: Detroit Black, Metro Region Black, Detroit White, and Metro Region White. In the charts that follow, each group has its own consistent symbol:

(1) Green diamond: Detroit Black

(2) Blue square: Metro Region Black

(3) Red triangle: Detroit White

(4) Green square: Metro Region White

On some of the charts it will appear as though only three groups are represented. This happens when two groups have the same value, resulting in overlapping symbols or when the number of births is so small that the percentages would be unreliable, in which case we’ve removed the symbol.

Mother’s Age, Race, and Place of Residence

Figure 1 and Table 1 illustrate that across the age groups, Blacks had higher rates of LBW infants than Whites in both Detroit and the Metro Region, with a slight advantage overall for Blacks residing in the Metro Region rather than in Detroit.

Figure 1 also shows a common pattern among three of the groups – rates that are high for teens, dip down for women in their 20s, and then get worse as mothers get older. Detroit Blacks, Detroit Whites, and Metro Region Blacks had increasing rates of low birth weight babies from ages 20-24 or 25-29 on.3 Particularly striking is the percentage of low birth weight babies for Detroit Blacks ages 35 and older (18%).

However, there is a counter trend among Metro Region Whites. Their rate of low birth weight babies decreased as age increased until age 35 and above.

Table 1: Percentage of LBW singleton births by mother’s age, area of residence, and race, Detroit and Metro Region, 2010-2012

Figure 1

Mother’s Education, Race, and Place of Residence

In the second blog post we saw that the rate of low birth weight was related to educational attainment in the same way for women living in Detroit and women living in the Metro Region: namely, the greater the educational attainment, the lower the rate of low birth weight. And that relationship does not change when we add race to the analysis.

The additional information we get by controlling for race is that the LBW rates for Black women are essentially the same or nearly so whether the women live in Detroit or the Metro Region. Furthermore, the rates of LBW are consistently higher for Blacks than for Whites in both areas4. It is also the case that the LBW rates for Detroit Whites are slightly higher than for Metro Region Whites but are closer to Metro Region Whites’ rates than to the LBW rates for Metro Region Black women.

Table 2: Percentage of LBW singleton births by mother’s education, area of residence, and race, Detroit and Metro Region, 2010-2012

Figure 2

Mother’s Marital Status, Race, and Place of Residence

In the first two blog posts, we showed that married women in Detroit and the Metro Region had lower rates of LBW than never married women. However, Figure 3 and Table 3 illustrate that among married women, the rates of low birth weight were higher for Black women than for White women regardless of area of residence.

Among never married women, Blacks had nearly identical LBW rates whether they lived in Detroit or the Metro Region. On the other hand, never married Detroit White women had a somewhat higher rate than Metro Region White women, placing them midway between Black women and Metro Region White women, a pattern we saw above with age and education.

Mother’s Adequacy of Prenatal Care, Race, and Place of Residence

The relationship between adequacy of prenatal care and birth weight follows the same pattern we have seen with age, education, and marital status: Black LBW rates are higher than White rates across all three levels of prenatal care adequacy; Black rates in the two regions are more similar to each other than to Whites’ rates; and Detroit Whites’ LBW rates lie midway between the Metro Region’s White rates and Black rates. Note that the gap between Blacks and Whites’ rates is greatest for adequate care (Figure 4 and Table 4). Also observe that an intermediate level of prenatal care raises the likelihood of a low birth weight baby relative to an adequate level of care more for Whites than for Blacks. It is an inadequate level of care, however, that is particularly detrimental to Blacks.

Table 4: Percentage of LBW singleton births by adequacy of prenatal care, area of residence, and race, Detroit and Metro Region, 2010-2012

Figure 4

Summary and Future Directions

We began this study of influences on babies’ birth weight with the knowledge that low birth weight is closely related to an increased risk of infant mortality, an especially pressing problem in Detroit. We wanted to know whether there are some relatively easily identified demographic characteristics of the mother that are associated with the likelihood of having a low birth weight baby. The Michigan birth certificate contains a number of such characteristics. We selected seven for our investigation: the mother’s age, education, marital status, ethnicity, and race; the level of prenatal care she received; and her area of residence (Detroit or the Metro Region).

This third blog post builds on the first two and adds a third layer of analysis. It is clear that a major difference between Detroit and the Metro Region is the racial composition of the populations. It is equally clear that one’s race can have profound implications for one’s well-being and opportunity. So the question became, “Would knowing a woman’s race increase our knowledge of the likelihood of her having a low birth weight baby beyond what we know from her other demographic characteristics and her area of residence? (For reasons of sample size, we restricted this analysis to women classified on the birth certificate as either Black or White.)

For the analysis we constructed four place-race groups: Detroit Blacks; Detroit Whites; Metro Region Blacks; and Metro Region Whites. Whether we examined the relationships by women’s age, education, marital status, or level of prenatal care, the results were the same: Blacks consistently had higher rates of low birth weight for their babies than Whites, whether they lived in Detroit or the Metro Region. Detroit Whites occupied an intermediate position, generally having somewhat higher rates than Metro Region Whites but typically lower rates than Black women in either region. In short, both place and race were significant factors for a baby’s birth weight.

However, it appeared that race was more influential than place for the LBW rate among Black women. This was particularly the case when we looked at the rate by educational attainment and level of prenatal care. For these two variables, Black women’s rates were very similar whether the women lived in Detroit or the Metro Region. Place seemed to play a more influential role for White women than for Black women, since Metro Region Whites generally had lower LBW rates than Detroit Whites.

A word of caution: The results reviewed in these three blog posts were intended only to stimulate conversation about demographic factors influencing birth weight, not to identify which of the factors were most important. In this initial investigation, we chose to examine the influence of selected demographic characteristics one at a time. The included characteristics are ones that are recorded on the birth certificate, but there are many other factors such as household income that we have not included in our analyses. Some of these were not included because the data do not exist on the birth certificate or if they are on the birth certificate, are unreliable. Other measures such as the mother’s health were excluded as being outside our area of interest.

Looking ahead, there are many other analyses to conduct that will give us a more nuanced understanding of variations in birth weight. A more insightful analysis, for example, will examine the simultaneous influence of several variables on birth weight. We may know that educational attainment is related to birth weight in a certain way and even know how the relationship is moderated by race and place of residence. But if we really want to be able to identify women most likely to have LBW babies, we will want to know which factors are most predictive, and that means we will need to consider multiple factors simultaneously. This will be the thrust of a future investigation.

Very low birth weight is defined as less than 1500 grams. In this analysis, very low and low are aggregated as “low” birth weight. ↩